When an image forming apparatus for a location where a voltage of a commercial power supply of a 100 V series (e.g., 100 V to 127 V) is used in a location supplying power for a 200 V series (e.g., 200 V to 240 V), the maximum power that can be supplied to a heater of a fixing section is quadrupled. When the maximum power that can be supplied to the heater becomes large, higher harmonic current, flicker or the like generated in heater power control, such as phase control or wave number control, becomes noticeable. Moreover, during the use in a location supplying 200 V, the power generated when the apparatus does not normally operate but thermally runs away increases four times as much as during its use in a location supplying 100 V, and hence a quick response safety circuit is required. Therefore, the heater is frequently replaced with a heater having a different resistance value for each location, so that the one image forming apparatus can be used in both a location where the commercial power supply voltage is 100 V and a location where the voltage is 200 V.
On the other hand, there is suggested a method of switching the resistance value of the heater by use of switch means, such as a relay, as means for realizing a universal image forming apparatus which can be used in both a location where the commercial power supply voltage of 100 V is supplied and a location where the commercial power supply voltage of 200 V is supplied. In Japanese Patent Application Laid-Open No. H07-199702 and U.S. Pat. No. 5,229,577, there is suggested an apparatus including first and second heat generating members, and it is possible to switch to a first operation condition where the first and second heat generating members are connected in series or a second operation condition where the members are connected in parallel, whereby the resistance value of each heat generating member is switched in accordance with a commercial power supply voltage, so that the commercial power supply voltage can be used in either of a location supplying 100 V and a location supplying 200 V.
In a method of switching the first and second heat generating members to a series connection condition and a parallel connection condition in accordance with the commercial power supply voltage, the resistance value of the heater can be switched without changing the heat generation area of the heater. In other words, the two heat generating members generate heat, when used in either of a location supplying 100 V and a location supplying 200 V. In a fixing apparatus including an endless belt, a heater which comes in contact with the inner surface of the endless belt, and a pressurizing roller which forms a fixing nip portion together with the heater via the endless belt, the above-mentioned series/parallel switching method is especially effective. This is because the two heat generating members generate the heat even when used in either of a location supplying 100 V and a location supplying 200 V, and hence a temperature distribution of the fixing nip portion in a recording material conveyance direction becomes the same irrespective of the location in which the apparatus is used. Therefore, there is the merit that the fixing properties of a toner image are not influenced by the location where the apparatus is used.
However, in the above method, when a detection section of the power supply voltage or a resistance value switch relay fails to operate properly, an excessively large power can be supplied to the heater sometimes. For example, while the image forming apparatus is connected to a commercial power supply of 200 V, the power which is four times as large as the power at a normal time that can be supplied to the heater in the parallel connection condition where the resistance value of the heater becomes low. Since the power supplied to the heater becomes excessively large, the response speed to interrupt the power supplied to the heater becomes insufficient sometimes in the safety circuit in which a temperature detection element such as a thermistor, a temperature fuse or a thermo SW is used. Therefore, in an image heating apparatus in which the resistance value can be switched, it is necessary to detect a failure condition where the large power is supplied to the heater by a method other than the method of detecting the temperature.
A purpose of the invention is to cope with various failure conditions of an apparatus in the apparatus in which first and second heat generating members can be switched to a series connection condition and a parallel connection condition.